Mass spectrometers are well known systems used for the detection and identification of chemical structures and quantitative elemental analysis of substances, In all known mass spectrometry methods, atoms or molecules to be sampled are excited and ionized, so as to create an ion beam, The ion beam is then accelerated through electric and magnetic fields to an ion collector, with the ion collector typically attached to an electrometer. The electrometer then translates signals received from the ion collector into a mass spectrum, which serves to indicate what elements (or radicals or fragments) are contained within the sample.
Many techniques have been suggested to excite and ionize the sample molecules and to fragment the ions from these molecules. These include the use of electrons to bombard species present in the gas phase, such as electron ionization; proton transfer reactions, such as those used in chemical ionization; or photoionization with lasers or other intense light sources. More recently, ionization has been accomplished by the use of metastable atom bombardment, in a which a neutral metastable species is used to bombard the sample molecules and fragment ions from these molecules. The use of metastable atom bombardment in ionizing the sample molecules has allowed the possibility of performing selective ionization, and control over the fragmentation of particles from the sample molecules. However, in order to perform metastable atom bombardment which consistently ionizes the sample material, a reaction mechanism is needed to produce a consistent source of metastable atoms, which is high in its intensity, charge free and low velocity.
A reaction system which produces a beam of metastable atoms is known in the art, and includes a reaction vessel having a source of rare gas at one end of the vessel, a cathode positioned inside the vessel and a small sonic nozzle placed at the other end of the vessel. Outside the vessel is a generally cone shaped anode referred to as a "skimmer" and which further includes an aperture at the apex of the cone. Behind the skimmer is a set of plates which serve as a deflector. In operation, the gas is detected at one end of the vessel and passes through the nozzle at the opposite end. The cathode within the vessel and the anode outside of the vessel are charged by a DC supply, such that a plasma arc is created between the cathode and anode. The atoms of gas which are injected through the discharge are energized to a metastable state, with some of the gas atoms being energized to the point of ionization, thus releasing free ions and electrons into the metastable gas stream. The metastable gas, the free ions and electrons then pass through the aperture in the apex of the skimmer into a set of charged deflector plates, where the free ions/electrons are attracted to the deflector plates, leaving the relatively charge free, metastable gas particles to pass through the deflector plates where it is used to bombard the sample substance to be analyzed by the mass spectroscopy apparatus.
A known disadvantage of this prior art device is that it does not always produce a consistent stream of metastable particles, and sometimes creates a stream of metastable particles mixed with ions/electrons. This occurs because the electric field which surrounds the cathode and anode is symmetric with respect to a longitudinal axis passing through the cathode and anode. As a result of this symmetric electrical field, the forces applied to the ions/electrons and ionized atoms created by the discharge is such that these particles are forced towards this longitudinal axis. Since this longitudinal axis also coincides with the axis of flow, the ions/electrons tend to remain in the flow path along with the metastable gas particles. Although the deflector does remove some of these ionized particles, the forces applied by the symmetric electric field work against the forces applied by the deflector, and thus ions tend to remain within the particle flow. Thus, the prior art apparatus does not produce a beam of purely metastable atoms, and produces spurious, unpredictable results when such a beam is used to ionize the sample to be tested by spectroscopy. The use of a skimmer and deflector plates also results in a larger assembly that causes a loss of metastable atoms. Because of the advantages of using metastable atom bombardment for selective ionization of the sample material, a need exists to improve the metastable atom bombardment system so that the beam of metastable atoms projected against the sample material only contains metastable atoms with a high density.